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Answer

as long as the signal is still strong enough, this should work. It’s no different to splitting sound sources really. Just make a Y-cable and extend, making sure you have a decent cross-section for long runs. Might be better to use bnc / xlr cables and convert back near the gopro. Those thin headphone extension cables are stiff , thin and 3,5mm jack plugs are simply nasty. Depending on the gopro model, you might also need a jack to usb adapter ( Hero 3 (+) )

Also experiment with the output signal level of the tentacle to make sure it can cover long runs. I would definitely take it a notch or two above the mic level for starters and see how far you can go before the gopro starts to clip, and then back off a couple of notches again.